Abstract
The dorsal cochlear nucleus (DCN) is the first site of multisensory integration in the auditory pathway of mammals. The DCN circuit integrates non-auditory information, such as head and ear position, with auditory signals, and this convergence may contribute to the ability to localize sound sources or to suppress perceptions of self-generated sounds. Several extrinsic sources of these non-auditory signals have been described in various species, and among these are first- and second-order trigeminal axonal projections. Trigeminal sensory signals from the face and ears could provide the non-auditory information that the DCN requires for its role in sound source localization and cancelation of self-generated sounds, for example, head and ear position or mouth movements that could predict the production of chewing or licking sounds. There is evidence for these axonal projections in guinea pigs and rats, although the size of the pathway is smaller than might be expected for a function essential for a prey animals’ survival. However, evidence for these projections in mice, an increasingly important species in auditory neuroscience, is lacking, raising questions about the universality of such proposed functions. We therefore investigated the presence of trigeminal projections to the DCN in mice, using viral and transgenic approaches. We found that the spinal trigeminal nucleus indeed projects to DCN, targeting granule cells and unipolar brush cells. However, direct axonal projections from the trigeminal ganglion itself were undetectable. Thus, secondary brainstem sources carry non-auditory signals to the DCN in mice that could provide a processed trigeminal signal to the DCN, but primary trigeminal afferents are not integrated directly by DCN.
Highlights
Accurate sound localization is essential for an animal’s survival and much of the auditory brainstem is specialized for this function
We found that the spinal trigeminal nucleus projects to dorsal cochlear nucleus (DCN), targeting granule cells and unipolar brush cells
AAV1-Syn-Cre was injected into the spinal trigeminal nucleus (SpV) of Ai9 mice (Figure 1A), and the axonal projections of the infected neurons were traced to their postsynaptic partners, as shown by clearly labeled tdTomato positive fibers and cell bodies in the facial motor nucleus, superior colliculus, and thalamus (Figures 1B–D)
Summary
Accurate sound localization is essential for an animal’s survival and much of the auditory brainstem is specialized for this function. Somatosensory signals from the head and face that could inform the auditory system of the current position of the jaw and ears – especially relevant to sounds source localization in animals with. The second-order neurons extend their axons to the contralateral thalamus along the trigeminal lemniscus. Disruption of this pathway may underlie some forms of tinnitus, the phantom percept of high-frequency sound commonly referred to as “ringing” of the ears. In 80% of tinnitus patients, head, jaw, and neck movements can modulate the perception of tinnitus (Levine et al, 2003) These movements cause altered activity in DCN (Lanting et al, 2010), possibly by enhancing somatosensory input via the trigeminal pathway. Identifying the neurons involved in carrying these trigeminal signals to the auditory system will be crucial to understanding the neural mechanisms of somatic tinnitus
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